Optical waveguides – Integrated optical circuit
Reexamination Certificate
2000-04-03
2001-04-24
Ullah, Akm E. (Department: 2874)
Optical waveguides
Integrated optical circuit
Reexamination Certificate
active
06222951
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a communication system utilizing optical fibers, and more particularly to a unit that connects individual computers to the fiber network system. The components for receiving the optical signals from and transmitting the optical signals to the fiber network system are fabricated in a batch-process fashion and integrated monolithically into the silicon-based electronic integrated circuits.
2. Description of Related Arts
Due primarily to their low loss and high-bandwidth, optical fibers are utilized in a wide variety of communication systems such as long-haul fiber systems. The recent advance in the fiber communication includes the construction of a fiber-to-the-fence infrastructure, promising the delivery of a large amount of information to homes and offices. However, the relatively expensive terminal equipment, such as a highly stable laser source required for transmitting the light signal, restricts the usage to central stations and prohibits the commercialization of a fiber network system that can download information from the network to the local user and further deliver information from the local user to the network.
One of the possible approaches to solve this cost dilemma is to provide the light source from a central station to individual customers, and split the incoming light from the fiber into two portions at the customer location. One portion is detected and converted to electrical signal and then processed by the local computer. Another portion of the incoming light is modulated by electrical signals from the local computer and then feed back to the fiber network system. Such a system will enable the connection of a computer to the network system with high speed and high data throughput capabilities.
Nevertheless, optical networks utilizing the most frequently used light wavelengths (i.e., 1.3 &mgr;m and 1.55 &mgr;m wavelength for low loss transmission) normally require the network unit be built from light sensitive materials such as GaAs or InP at the corresponding wavelengths. Those materials are relatively expensive as compared to the popular silicon substrate. Moreover, the device fabrication processes for GaAs and InP are not compatible with the existing mature silicon integrated circuit technology, which serves as the backbone for the fast-growing electronic computer industry.
Although silicon chips have the advantage of low cost and easy processing, silicon material alone cannot response to the light wavelength used in the long-haul fiber system. In order to build individual devices from GaAs, InP or other silicon-based materials such as silicon-germanium or silicon-germanium-carbon to form optical splitters, photodetectors, optical modulators and electrical amplifiers required by the optical network unit, put them together onto a silicon integrated circuit chip and will yield a bulky and costly unit. It is therefore highly desirable to integrate the electrical and optical components of the network unit with the integrated circuit chip in the same silicon substrate. The trend to move towards silicon-based silicon-germanium optical integrated circuit for optical network unit can be seen in the prior art.
U.S. Pat. No. 4,754,452 to Henry describes an optical local area network system wherein one carrier source is shared by a cascade of users in the system.
U.S. Pat. No. 5,577,139 and No. 5,577,138 to Chandrasekhar et al (hereafter referred to as the prior art) disclose an integrated-circuit optical network unit wherein the unit is made in an integrated-circuit form based on In
0.53
Ga
0.47
As and In
0.65
Ga
0.35
As
0.7
P
0.3
compound materials system lattice matched to the InP substrate.
Since InP-based InGaAs compound has a direct bandgap of energy below 1.55 &mgr;m wavelength, the absorption coefficient is large; and a small InGaAs thickness on the order of a micron meter will yield a sufficient light absorption at this wavelength. As a result, the InP-based network unit can be built in a surface normal fashion, wherein the incoming and outgoing optical waves are perpendicular to the surface of the substrate material.
Silicon-based materials, on the other hand, suffer several difficulties in constructing a similar network unit using the previous art. Firstly, silicon-based materials are nearly transparent to optical light at 1.55 &mgr;m wavelength. As a result, other alloys such as silicon-germanium with a narrower bandgap have to be incorporated. Due to the lattice mismatch of silicon-germanium alloy from the silicon substrate, it is extremely hard, if not totally impossible, to grow high quality silicon-germanium alloy with a large thickness of several microns. Secondly, due to the small optical absorption coefficient of silicon-germanium alloy at 1.55 &mgr;m, a waveguide structure has to be used to increase the absorption length. Therefore, a surface-normal form of the network unit as described in the prior art based on the InP system is no longer applicable in the silicon system.
Although silicon-based optoelectronic devices such as photodetectors and optical modulators have been demonstrate, and optical receiver system with a certain degree of integration into electronic circuitry has been reported, an integrated circuits designed specifically for an optical network unit operating at the 1.3 &mgr;m and 1.55 &mgr;m wavelengths has never been disclosed.
U.S. Pat. No. 4,.426,440 to Thompson devices an integrated optical grating by thermal SiO
2
grown on Si.
U.S. Pat. No. 4,787,691 to Lorenzo et al describes all silicon electrooptical devices for modulating and switching of guided light wherein a silicon-on-insulator approach has been used.
U.S. Pat. No. 4,789,642 and U.S. Pat. No. 4,857,973 to Soref et al disclose a method of fabricating low loss crystalline silicon waveguide by dielectric implantation, and a method of integration with a Schottky barrier photodetector.
U.S. Pat. No. 4,787,691 to Soref et al describes an optical modulator based on a silicon-on-sapphire substrate. This modulator applies the free carrier effect to achieve modulation of light in the 1.3-1.55 &mgr;m wavelength range by injecting external current to the waveguide.
Some publications related to major developments in silicon-based optoelectronic devices are also included here for general reference purposes. “A self-aligned SiGe base bipolar technology using cold wall UHV-CVD and its application to optical communication ICs,” by Sato, et al., IEEE Trans. Electronic Devices, vol.42, p.82-88 1995, described a silicon-based electronic amplifier. A book chapter entitled “Si-based superlattices: Photonic applications,” in
Handhook of Thin Film Process Technology,
(IOP Publishing Ltd, England, 1997), chapter F6., by Huang and Jalali presented an overview of the related art.
As compared to the optical network unit disclosed by the prior art using InP based-material system, the silicon-based optical network unit to be disclosed in the present art has the unique feature of being able to integrate into the silicon electronic integrated circuits (ICs), which promises huge cost reduction as compared to the InP-based systems. IBM Corporation has commercialized silicon-germanium heterojunction bipolar transistors (HBTs) monolithically integrated with silicon complimentary metal oxide semiconductor (CMOS) circuits. The silicon-germanium HBTs can operate much faster than the CMOS amplifier, and thus can be served as the amplifier device needed for high speed application in the present invention.
The aforementioned silicon-based monolithically integrated fiber network units, with superior features of low cost, high reliability, and easy in integration with the silicon integrated circuits, will provide a building block for high speed and affordable broadband optical network system accessible by small network service providers or even individual computer users.
SUMMARY OF THE INVENTION
It is therefore an objective of the present invention to provide an optical network unit comprising silicon-germanium electronic a
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